Electron discharge tube



Oct. 31, 1939. M K TAYLOR ELECTRON DISCHARGE TUBE Filed Sept. 1, 1957 Patented Oct. 31, 1939 UNITED STATES PATENT OFFICE Application September signer to Ferranti Electric 1110., New York, N. Y.

1, 1937, Serial No. 162,034

In Great Britain September 2, 1936 2 Claims.

This invention relates to electron discharge tubes of the type wherein electrons emitted by a source impinge upon a secondary-emitting electrode from which secondary electrons are collected by an anode at a higher potential than the source, the number of electrons passing from the cathode to the anode at any instant being dependent, among other things, upon the potential of a grid electrode interposed between the cathode and the anode.

In circuits incorporating such a tube, e. g. a thermionic valve, it is usually desirable to utilise a tube having as high a mutual conductance as possible. However, a high mutual conductance f entails a high manufacturing cost due to the greater care and accuracy needed to ensure a given uniformity of product.

An object of the present invention is to provide improved forms of electron discharge tubes having a high value of mutual conductance without the necessity for limits of tolerance so close that the product becomes excessively difficult and costly to manufacture.

The invention comprises an electron discharge tube having features as set forth in the claims appended hereto.

Referring to the accompanying diagrammatic drawing: I

Figure 1 illustrates one convenient form of electron discharge tube embodying the invention, certain parts being broken away in order, more clearly, to illustrate the construction.

Figure 2 is a sectional planview thereof.

Figure 3 is a sectional plan view of a slightly modified construction.

Figure 4.- is a sectional plan View of a further modified arrangement.

Figure 5 shows a modified form of a detail.

In carrying the invention into effect according to one form by way of example, I use as a source of electrons a thermionic cathode a. Surrounding the cathode is a control grid b wound on support wires 0, which grid in turn is surrounded by a screen grid d. An anode formed as two elements 6, disposed at opposite ends of a diameter passing through the cathode lies outside the latter grid, the two elements being electrically connected together by a wire f. Disposed substantially in the circumferential spaces between the two elements e, of the anode are two electrodes 9, electrically connected together by a wire h, these electrodes being coated by well-known methods on their inner surfaces with material that emits electrons under the action of electron bombardment. The electrode assembly is mounted on a foot in an evacuated envelope 2' in the usual manner, leads being brought out through a press 1' so that potentials may be applied to the various electrodes. A coil is surrounds the complete tube.

In operation a direct current is passed through the coil It so generating a steady magnetic field which traverses the tube in a direction parallel to the axis of the electrodes. A high positive potential is applied to the anode e, a lower positive potential to the secondary emissive electrode g, a still lower positive potential to the screen grid at and a controlling potential together with the desired biasing potential is applied to the control grid 1). I

The approximate paths of primary electrons and secondary electrons are indicated in Figure 2 by dotted lines and chain-dotted lines respectively.

Primary electrons are emitted by the cathode a, being partially constrained to follow two fanshaped paths between the control grid support wires 0. The superimposed magnetic field is in such a direction that in co-operation with the potential of the electrode g, the fan-shaped streams of electrons impinge on the elements of the electrode 9, with sufficient energy to liberate a larger number of secondary electrons. These secondary electrons under theaction of the potential gradients and the magnetic field, are then collected by the elements e, of the anode, and the 30 corresponding output current may be used to operate apparatus in the anode circuit of the tube.

'The number of primary electrons incident at any instant upon the secondary emissive electrode g, is dependent, among other factors, on the potential of the control grid b. Since the number of electrons reaching the anode is proportional to but larger than the number of primary electrons, it is evident that a larger value of mutual conductance is obtained than would be obtained 40 without the use of the secondary emissive electrode.

In a modification illustrated in Figure 3, instead of utilising the grid support wires 0, to concentrate the electrons into a fan-shaped beam, I employ additional electrodes m, at a small negative potential for this purpose. These may be either narrow strips of sheet-metal as shown or straight wires of circular cross-section.

According to a further modification the elements g, of the secondary emissive electrode are formed as a coating of secondary emissive material on the inner wall of the containing envelope 2', which must be of insulating material, e. g. glass. If desired the elements e, of the anode may be formed as coatings of conductive material on the inner wall of the containing envelope i, which must be of insulating material, e. g. glass. These arrangements result, in certain cases, in a cheapening and simplification of the assembly.

When the tube is used to control currents of comparatively great magnitude, for example in a tube used as a power valve in an amplifier, it is necessary to prevent the temperature of the secondary emissive electrode rising to a value at which thermionic emission commences. For this purpose I may either form the secondary emissive electrode in such a manner as to have a surface of large radiating area, such surface being treated with a black finish, or alternatively I may as illustrated in Figure 5 afiix cooling portions 11 to it, for example by welding radiating fins to the surface remote from the cathode so as to be out of the line of flight of electrons.

Although the invention has been described as applied to a. valve of the screen grid type it will be appreciated that it can be applied to other types, for example, any multi-grid valve. Moreover instead of a thermionic cathode, a photoemissive cathode may be used.

When a positive potential is applied to the secondary emissive electrode, more electrons are ejected by it than are incident upon it. Further if its positive potential is increased a still larger number of electrons is ejected, with the result that if a tuned circuit of suitable impedance is associated with the secondary emissive electrode, oscillations are generated. The tube may therefore be used as a mixing valve in a superheterodyne receiver.

A further application of a discharge tube embodying the present invention is to the amplification of television signals when it is desired to amplify substantially uniformly a very large range of frequencies. For this purpose the secondary emissive electrode is used as the anode, so that it becomes part of the output circuit. The

utual conductance of the valve with respect to this electrode is negative with the consequence that whereas feed-back through the valve capacity usually tends to cancel the input signal, in this application feed-back actually increases the input signal. Hence an amplifier using these valves may be designed and constructed in which substantial compensation for input and output capacities is obtained. Higher values of coupling resistances than are usual may thus be utilised without resultant loss of higher frequencies, and these high values together with the high numerical value of mutual conductance of the discharge tube enable a large amplification to be obtained.

A further application of this type of discharge tube is for gain control or modulation by variation of the strength of the magnetic field applied to the tube. This causes more or fewer electrons to reach the anode thus varying the anode current.

In some applications of gain control, c. g. in A. V. C. circuits, it is useful to be able to vary the gain of an amplifying valve without producing a change in the value of the anode current of the valve. The discharge tube herein described enables this to be accomplished in the following manner. Over the working range, the control electrode has a substantially linear anode current-control voltage characteristic. Variation of the magnetic field causes a variation of the anode current and of the gain; if therefore a change is made in the magnetic field, accompanied by a change of the control electrode potential in such a direction that the anode current remains unaltered-in value, gain control without alteration of anode current is obtained.

As an alternative to an electromagnet for providing the magnetic field a permanent magnet in the form of an annulus may be employed.

I claim:

1. An electron discharge tube electrode assembly comprising a cathode, a control electrode, a secondary-emitting electrode formed in sections and an anode disposed within the sphere of action of secondary electrons from the secondaryemitting electrode and formed in sections, the said sections of the secondary-emitting electrode and those of the anode lying in successive disposition on a common cylindrical surface of substantially circular cross-section.

2. An electron discharge tube electrode assembly as claimed in claim 1, including also additional electron concentrating electrodes.

MAURICE KENYON TAYLOR. 

